Actress/director Sarah Polley detailed how senility shattered a long-married couple in her directorial debut, the 2006 film “Away From Her.”

Now, Polley is moving in a more audacious direction, examining what happens when a perfectly happy married woman meets a man with whom she has a laboratory full of chemistry?

It’s a love triangle without overt villains, a story with a love struck couple stretching marital bonds to the breaking point while never dismissing the institution or its cultural value.

“Take This Waltz” may be challenging, but it’s not with structural defects. Polley’s dialogue can be cloying and overstated – that is, when the situations aren’t comically obvious and manipulative. Michelle Williams’ latest bravura turn holds this triangle together, letting us feel her anguish and longing in ways that tug at our hearts while her character comes to a series of shattering realizations.

Margot and Lou (Williams and Seth Rogen) live a contented life in a warmly captured Toronto neighborhood. They tease each other with violent jokes, trot out funny accents as part of their foreplay rituals and live to snuggle themselves to sleep.

Margot meets a handsome man (Luke Kirby) at such a park, and the two immediately share a warm, fizzy chemistry. They later sit next to each other on a plane (rom-com coincidence!) where their sexually charged bickering continues. It’s instant attraction, and when they part she sheepishly tells him she’s married, and both parties assume that will be that.

But Daniel just so happens to live a few feet away from Margot’s house, and when he hauls out his human-powered rickshaw each morning (precious alert!) she can’t help but rise early to see him off.

What follows is a tentative relationship made of stolen moments and not so chance encounters. They both know it’s wrong, but proximity and curiousity win out nearly every time.

Williams began her career on “Dawson’s Creek,” the kind of potboiler that leaves many actors wanting for work upon cancellation. She’s not a traditional beauty, yet her talent lies in laying her characters bare for our inspection. Watching her fall in love is to witness acting by degrees. You can see her heart flutter, hear her pulse quicken.

When Daniel describes what he would do to her should they ever dare to consummate their passions Williams melts before his eyes, and ours. It could be the most erotic movie moment of the year, even though no one removes so much as a sock or scarf.

Rogen puts aside his raunchy comedy mannerisms – although his staccato pot laugh remains – to make Lou the kind of man most woman would adore unconditionally. Will it be enough?

Polley’s story stumbles badly in the final moments, from a ridiculously arranged montage to a subplot lugging co-star Sarah Silverman’s character back into the picture to assist the resolution. The bruised feelings and tenderness Polley’s film generate remain, growing stronger during the thorny emotional climax.

“Take This Waltz” will linger In your thoughts, even as its tropes and imperfections quickly fade away.